In the art of flexographic color printing, photo-engraved photopolymer plates are used in multiple processes. The first step in preparing color printing plates is to make a camera film negative from camera ready art (mechanical art). Multiple color separated film negatives are then produced from the camera film with the aid of a mask for line processes, or with the aid of color filters for half tone processes. After the color separated negatives are developed, the color separated image is photographically transferred to a photosensitive plate, with a separate photopolymer printing plate being produced for each color. The exposed plate is then etched to define the printing surface. Non-printing areas on the plate are removed by the etch solution, leaving the printing surface standing in relief.
The finished plates are thereafter separately mounted onto print cylinders. The plates are inked and applied to separate sheets or a moving webb of printing paper or film, with the print fields of the various plates being superimposed upon one another. Thus, the picture is successively built up step-by-step, and when the impressions are properly superimposed, the resulting print is a faithful reproduction of the mechanical art.
To achieve good results, it is essential that the successive print fields register exactly one upon the other, with the separate color impressions being spaced with respect to each other so that overlapping does not occur. The complementary printing plates must be precisely aligned to obtain appropriate positioning of the multiple color impressions. Therefore, it is essential that the print fields of the flexographic plates be aligned within very close tolerances at corresponding locations on each print cylinder.